Self discharge, capacity maintenance, oxidation, and water loss problems in secondary iron-air batterie

Wagner, O. C.

English

NASA Technical Reports Server

)PIC-BA209/11
SECONDARY IRON-AIR BATTERIES
Otto C. Wagner
Power Sources Division, Electronic Components Laboratoz_
UBAECOM, Fort Monmouth, N.J. _ _ _ . -@
N c9-10g86
Ln order to attair a maximum energy density fr_ the iron air system, "''
non-supported active Iron-electrodesmust be developed. The theoretical
energy density of the iron anode for the Iron-to-ferrous state is 556
watt-hours per pound; for the iron-to-ferric state the total theoretical
energy density is 837 watt-hours per pound. To utilize the three electron
change the system will discharge on two voltage plateaus; namely,0.8-0.9
++ ++ @++
volts for the Fe/Fe reaction and 0.7-0.6 volts for the Fe /Fe reaction.
The two voltage plateaus are disadvantageous for most portable power applica-
tions, although the energy realized from the total reaction is very attractive.
Another major advantage of the alkaline iron system is that iron is
cheap and very abundant as a raw material. This factor is of greatest
importance in addition to its high theoretical energy density. The long
cycle life of the alkaline iron anode (more than 20 years of service in
Edison Ni/Fe batteries, for example) provides another attractive feature
for this system.
An important limitation of the iron-air system, employing the con-
ventlonal iron anodes, is that its shelf llfe is very poor. This is due to
the low hydrogen overpotentlal of the electrode. In addition, the con-
ventional iron electrode requires 50-60 percent overcharge to attain
maximum electrochemical utilization of the active material. This results
in an appreciable loss of water, which means that a conventional alkaline
Iron-system would require considerable maintenance.
• Approaches to overcome the limitations of the alkaline iron system
.' have been reported by many investigators, and it appears that an Iron-alr
- and/or other alkaline iron systems can be developed to meet the needs of
present day portable power requirements.
" The major failure modes reported in the literature for the iron-alr
system are: (a) self-discharge, (b) oxidation of the anode, (c) loss of
active materi_l utilization on deep discharge and reverse discharge and
(d) water loss.
The first topic is Self Discharge - The self discharge of the iron
electrode is mainly caused by the couple action between the low hydrogen
overpotential impurities on the electrode and the active iron powder. (1)
Burshtein reports that this seLf-discharge can be reduced to minimal rate
by partially passivating the electrode by treating it with benzene or
'_i. other volatile organic liquids (2). It is also reported that the electrode
can be beneficially treated by the addition of i% water glass or sodium
_, phosphate inhlbitors to the electrolyte.
1969000953-017
https://ntrs.nasa.gov/search.jsp?R=19690000955 2020-03-12T05:28:00+00:00Z
PIC-BAT 209/11
The self-discharge of the alkaline iron electrode is accelerated by
the employment of nickel powder conductor material or other low hydrogen
overpotential materials. Therefore, to provide the electrode with a
conductive matrix which does not induce self discharge it is necessary to
employ materials (in addition to the conductive Fe of the electrode itself)
with hydrogen overpotentialshigher than that of alkaline iron. In addition,
the conductor must not form resistive oxide layers, or dissolve in the
electrolyte. Mercury.and copper have been beneficially employed in the
conventional iron electrodes of the pocket or tubular types. Since these
metals possess hydrogen overpotentials in alkaline electrolytes higher
than that of iron it is possible that they provide the desired conductive
matrix. Other effects such as extender or_expander action are also
possible.
The second topic is Oxidation of the Anode - Oxidation of the anode
by air is a problem common with all metal-air systems. Methods to over-
come this problem are: cladding the anode with semipermeable membranes
and constructing the unit cell so that air can have access to the system only
through the pores of the air cathode; that is, the unit cells must be de-
signed as sealed structures.
The third topic is Loss of Active Material Utilization - It is known
that pure alkaline iron systems, where the iron electrode is limiting on
discharge, lose capacity by irreversible electrochemical reactions. It is
reported in the literature that the active material utilization of iron
powder is stabilized and increased by the use of graphite (3), molybdenum
(4), tungsten (4), and sulfur additives. These additives may be beneficial by
reducing the self-discharge reaction of the iron anode and/or providing
expander and/or extender action.
Water Loss - The problem of water loss is common for all metal-air
systems. For iron-air it is serious because of the self-discharge
reaction and the need for extensive overcharge. Methods to minimize this
problem are: (a) reducing the self-discharge reaction of alkaline iron
(as previously discussed), (b) improving the active material utilization
of the iron anode so that it is highly reversible and (c) providing the
unit iron-air cell with a reserve of excess electrolyte in a reservoir.
Performance Characteristics of the Iron-Air System- General Telephone
and Electronics has been pioneering the development _" a practical iron-air
system during the past several years. They report that they have developed
iron-air cells of 5 to 20 ampere-hour capacities with a cycle life greater
than 200 cycles at energy densities of.60-70 watt hour/pound. The regime
was at 65 percent depth of discharge at the C/2 rate (5). As with the ECC_
cadmium-air system the air cathode of the GT&E system was a bifunctional
platinum electrode.
Up to the present time, R&D efforts at ECOM with the Iron-air system
have been very limited. Explor,_torywork has been done with iron electrodes
supplied by GT_E and air cathodes by Lmesona Moos. Data _av_e_1_eene_rati6 _
and nonconclusive. Work will be carried out in the near _'utureutilizing
the knowledge of the present state-of-the-artof the electrodes as well as
novel approaches derived from internal work.
-l_-
1969000953-018
PIC-BA2o9/11
Conclusion - If the seLf-discharge,capacity maintenance, oxidation and
water loss problems of the iron-air couple are resolved, this system should
become very attractive as a low cost, high energy density power source for
communication and transportation applications.
References
i. Electrode Processes In New ELectrochemical Sources of Current; V. S.
Bagotski; Conference Moskau 1956; Pgs. 737-43.
2. The Iron-Carbon Cell; N A. Shurmorskaia and R. K. Burshteln; Conference Moskau
1956; Pgs. 768-72.
3. Cathodic Reduction Of Iron; V. P. Galushko, E F. Zavgorodnaya and L. I.
Tishchenko; Zhur. Priklad Khim; 1959, 32, 1551-1555.
4. Hydrogen Overpotential at porous Iron Electrodes Containing Certain
Alloying Metals (Cu,Ni,W,Co; and Mo); N. N. Voronin, I. B. Barmashenko,
and D. S. Nadeszhdin; Ukrain Khin. Zhur. 1954, 20, 182-193.
5. Survey And Analysis on Metal-Air Cells; A. Fleischer; Tech Report
AFAPL-TR-68-6; March 1968; Pgs. 27-28.
-15-
1969000953-019


Secondary iron-air batteries

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server